The present invention relates to a circuit for selecting a channel in a communication (transmission-receiving) system having a plurality of channels such as a Time Division Multiple Access (TDMA) system. More particularly, the present invention is directed to a communication channel selecting circuit corresponding to radio signal intensity.
A Personal Handyphone system (PHS) adopted Time Division Multiple Access-Time Division Duplex (TDMA-TDD) system selects one channel from four channels, from CH1 to CH4 (4 channels for receiving and 4 channels for transmission) and performs transmission and receiving. ln this TDMA-TDD system, a radio signal received from an antenna is demodulated at a receiving timing, and data is reproduced. To confirm whether the radio signal is able to be demodulated to reproduce the data without deficiencies and has sufficient intensity or not, intensity of a signal, indicating radio signal intensity converted from a radio signal, is measured/monitored at a receiving timing of each of the channels from channel 1 to channel 4.
The intensity of the signal indicating radio signal intensity is judged at the Central Processing Unit (CPU), the CPU selects a receiving channel indicating the highest intensity and, at the same time, it also selects a transmission channel corresponding to the selected receiving channel. A communication channel is thus selected in the conventional system.
The conventional channel selection process by measuring the intensity of the radio signal as described above does however have problems as outlined in the following.
In this instance, the problem is described with reference, as an example, to a cordless telephone system handset of which can transmit and receive information by radio signal. When power is applied to a main telephone of cordless telephone system or the system is reset, the operation starts with a timing of the main telephone. The main telephone, starting its operation, measures the intensity of radio signals at its own receiving timing, and selects the most intensive receiving channel. At the time of measuring, if another similar cordless telephone system is in use nearby, the main telephone, newly applying power or resetting the system, measures the radio signal intensity of the channel used by another similar cordless telephone system, and selects the same or adjacent channel used by another similar cordless telephone system.
Accordingly, transmitting the radio signal that overlaps with other radio signals in the channels already used may cause an undesirable disturbance to other radio apparatuses.